In recent years, advances in technology have led to substantial changes in the design of automobiles. One of these changes involves the complexity, as well as the power usage, of various electrical systems within automobiles, particularly alternative fuel vehicles. For example, alternative fuel vehicles such as hybrid vehicles often use electrochemical power sources, such as batteries, ultracapacitors, and fuel cells, to power the electric traction machines (including electric motors and motor/generators) that drive the wheels, sometimes in addition to another power source, such as an internal combustion engine.
Such electric machines typically include a rotor assembly that rotates axially on a shaft within a stationary stator. Because rotor assemblies of traction electric machines may rotate at thousands of revolutions per minute (rpm), minor anomalies in the balance of such assemblies can result in undesirable vibrations, premature wear of bearings and shafts, and reduced machine life. To compensate for such imperfections and maintain balance at high speeds, annular metallic balance rings having an asymmetric weight distribution are typically mounted on one or both ends of a rotor concentric to the shaft. The ring is machined as necessary to remove weight at specific points along the circumference of the ring customized for and complementary to the imbalances of a particular rotor. Ideally, balance rings are characterized by both high mechanical strength to withstand the forces generated when rotating at high rpm's, and low magnetic permeability (μ) to contain magnetic flux within the rotor assembly to improve overall machine efficiency. Balance rings fabricated as a unistructure from such metals as stainless steel offer both of these characteristics, but are prohibitively expensive. Materials such as aluminum are relatively inexpensive and offer high magnetic shielding but have insufficient mechanical strength for many applications. Other inexpensive materials such as carbon steel offer high mechanical strength but provide little or no magnetic shielding.
Composite balance rings fabricated as a bi-layer structure comprised of two different powdered metals have been used as a means of combining both high magnetic shielding and mechanical strength. For example, a layer of a first powdered metal having a low μ such as stainless steel, may be combined with a layer of a second powdered metal having low cost and high strength, such as carbon steel. The two layers are then sintered together to form a balance ring which is then machined for a particular rotor. However, metals in powder form are significantly more expensive than metals in other, more common forms such as rolled sheets. Further, sintered stainless steel tends to work harden during machining, making it an especially difficult material to process. In addition, sintered balance rings are subject to cracking during such machining. Such cracks may be difficult to identify during the manufacturing process, and thus may result in a failure of the ring when loaded at high speeds.
Accordingly, it is desirable to provide a balance ring for a vehicular electric machine that has both low magnetic permeability and high mechanical strength using inexpensive starting materials. Further, it is also desirable if such balance rings are easier to machine and less susceptible to forming cracks. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.